Cationic amphipathic peptides accumulate sialylated proteins and lipids in the plasma membrane of eukaryotic host cells

Sulfated Glycoaminoglycans and Proteoglycan Syndecan-4 Are Involved in Membrane Fixation of LL-37 and Its Pro-Migratory Effect in Breast Cancer Cells

Initially characterized by its antimicrobial activities, LL-37 has also been shown to significantly contribute to tumor development. On breast cancer cell lines, LL-37 increases intracellular calcium via the TRPV2 channel and their migration via the activation of PI3K/AKT signaling. Its all-d enantiomer d-LL-37 induces similar effects, which excludes a protein-protein interaction of LL-37 in a classic ligand-receptor manner. Its net charge of +6 gave rise to the hypothesis that the peptide uses the negative charges of sulfoglycans or sialic acids to facilitate its attachment to the cell membrane and to induce its activities. Whereas several vegetal lectins, specifically attaching to sialylated or sulfated structures, blocked the activities of LL-37 on both calcium increase and cell migration, several sialidases had no effect. However, the competitive use of free sulfated glycoaminoglycans (GAGs) as chrondroitin and heparin, or treatment of the cell surface with chondroitinase and heparinase resulted in an activity loss of 50–100% for LL-37. Concordant results were obtained by blocking the synthesis of GAGs with 4-Methylumbelliferyl-β-d-xyloside, and by suppression of glycan sulfatation by sodium chlorate. Using a candidate approach by suppressing proteoglycan synthesis using RNA interference, syndecan-4 was shown to be required for the activities of LL-37 and its binding to the cell surface. This leads to the conclusion that syndecan-4, by means of sulfated GAGs, could act as a receptor for LL-37.

Combining TIR and FRET in Molecular Test Systems

Pharmaceutical agents or drugs often have a pronounced impact on protein-protein interactions in cells, and in particular, cell membranes. Changes of molecular conformations as well as of intermolecular interactions may affect dipole-dipole interaction between chromophoric groups, which can be proven by measuring the Förster resonance energy transfer (FRET). If these chromophores are located within or in close proximity to the plasma membrane, they are excited preferentially by an evanescent electromagnetic wave upon total internal reflection (TIR) of an incident laser beam. For the TIR-FRET screening of larger cell collectives, we performed three separate steps: (1) setting up of a membrane associated test system for probing the interaction between the epidermal growth factor receptor (EGFR) and the growth factor receptor-bound protein 2; (2) use of the Epac-SH188 sensor for quantitative evaluation under the microscope; and (3) application of a TIR fluorescence reader to probe the interaction of GFP with Nile Red. In the first two steps, we measured FRET from cyan (CFP) to yellow fluorescent protein (YFP) by spectral analysis and fluorescence lifetime imaging (FLIM) upon illumination of whole cells (epi-illumination) as well as selective illumination of their plasma membranes by TIR. In particular, TIR excitation permitted FRET measurements with high sensitivity and low background. The Epac sensor showed a more rapid response to pharmaceutical agents, e.g., Forskolin or the A2B adenosine receptor agonist NECA, in close proximity to the plasma membrane compared to the cytosol. Finally, FRET from a membrane associated GFP to Nile Red was used to test a multi-well TIR fluorescence reader with simultaneous detection of a larger number of samples.

The toxic alpha-gliadin peptide 31-43 enters cells without a surface membrane receptor

Alpha-gliadin peptide 31-43 is considered to be the main peptide responsible for the innate immune response in celiac disease patients. Recent evidence indicates that peptide 31-43 rapidly enters cells and interacts with the early endocytic vesicular compartment. However, the mechanism of its uptake is not completely understood. Our aim is to characterize, isolate and identify possible cell surface proteins involved in peptide 31-43 internalization by Caco-2 cells. In this study, we used a chemical cross-linker to block peptide 31-43 on cell surface proteins, and pulled-down peptide-proteins complexes using antibodies raised against peptide 31-43. Through this experimental approach, we did not observe any specific complex between cell proteins and peptide 31-43 in Coomassie-stained denaturating gels or by Western blotting. We also found that type 2 transglutaminase was not necessary for peptide 31-43 internalization, even though it had a regulatory role in the process. Finally, we demonstrated that peptide 31-43 did not behave as a classical ligand, indeed the labeled peptide did not displace the unlabeled peptide in a competitive binding assay. On the basis of these findings and of previous evidence demonstrating that peptide 31-43 is able to interact with a membrane-like environment in vitro, we conclude that membrane composition and organization, rather than a specific receptor protein, may have a major role in peptide 31-43 internalization by cells.

The importance of the glycosylation of antimicrobial peptides: natural and synthetic approaches

Glycosylation is one of the most prevalent post-translational modifications of a protein, with a defining impact on its structure and function. Many of the proteins involved in the innate or adaptive immune response, including cytokines, chemokines, and antimicrobial peptides (AMPs), are glycosylated, contributing to their myriad activities. The current availability of synthetic coupling and glycoengineering technology makes it possible to customise the most beneficial glycan modifications for improved AMP stability, microbicidal potency, pathogen specificity, tissue or cell targeting, and immunomodulation.

Perspectives on polymeric nanostructures for the therapeutic application of antimicrobial peptides

Antimicrobial peptides (AMPs) are a class of promising anti-infective molecules but their therapeutic application is opposed by their poor bioavailability, susceptibility to protease degradation and potential toxicity. The advancement of nanoformulation technologies offers encouraging perspectives for the development of novel therapeutic strategies based on AMPs to treat antibiotic resistant microbial infections. Additionally, the use of polymers endowed per-se with antibacterial properties, stands out as an innovative approach for the development of a new generation of drug delivery systems in which an enhanced antimicrobial action could be obtained by the synergic combination of bioactive polymer matrices and drugs. Herein, the latest AMPs drug delivery research is discussed.

Biomolecular Characterization of Putative Antidiabetic Herbal Extracts

Induction of GLUT4 translocation in the absence of insulin is considered a key concept to decrease elevated blood glucose levels in diabetics. Due to the lack of pharmaceuticals that specifically increase the uptake of glucose from the blood circuit, application of natural compounds might be an alternative strategy. However, the effects and mechanisms of action remain unknown for many of those substances. For this study we investigated extracts prepared from seven different plants, which have been reported to exhibit anti-diabetic effects, for their GLUT4 translocation inducing properties. Quantitation of GLUT4 translocation was determined by total internal reflection fluorescence (TIRF) microscopy in insulin sensitive CHO-K1 cells and adipocytes. Two extracts prepared from purslane (Portulaca oleracea) and tindora (Coccinia grandis) were found to induce GLUT4 translocation, accompanied by an increase of intracellular glucose concentrations. Our results indicate that the PI3K pathway is mainly responsible for the respective translocation process. Atomic force microscopy was used to prove complete plasma membrane insertion. Furthermore, this approach suggested a compound mediated distribution of GLUT4 molecules in the plasma membrane similar to insulin stimulated conditions. Utilizing a fluorescent actin marker, TIRF measurements indicated an impact of purslane and tindora on actin remodeling as observed in insulin treated cells. Finally, in-ovo experiments suggested a significant reduction of blood glucose levels under tindora and purslane treated conditions in a living organism. In conclusion, this study confirms the anti-diabetic properties of tindora and purslane, which stimulate GLUT4 translocation in an insulin-like manner.

Identification of novel insulin mimetic drugs by quantitative total internal reflection fluorescence (TIRF) microscopy

Background and Purpose

Insulin stimulates the transport of glucose in target tissues by triggering the translocation of glucose transporter 4 (GLUT4) to the plasma membrane. Resistance to insulin, the major abnormality in type 2 diabetes, results in a decreased GLUT4 translocation efficiency. Thus, special attention is being paid to search for compounds that are able to enhance this translocation process in the absence of insulin.

Experimental Approach

Total internal reflection fluorescence (TIRF) microscopy was applied to quantify GLUT4 translocation in highly insulin-sensitive CHO-K1 cells expressing a GLUT4-myc-GFP fusion protein.

Key Results

Using our approach, we demonstrated GLUT4 translocation modulatory properties of selected substances and identified novel potential insulin mimetics. An increase in the TIRF signal was found to correlate with an elevated glucose uptake. Variations in the expression level of the human insulin receptor (hInsR) showed that the insulin mimetics identified stimulate GLUT4 translocation by a mechanism that is independent of the presence of the hInsR.

Conclusions and Implications

Taken together, the results indicate that TIRF microscopy is an excellent tool for the quantification of GLUT4 translocation and for identifying insulin mimetic drugs.

Role of glycosylation in the anticancer activity of antibacterial peptides against breast cancer cells

Antibacterial peptides (ABPs) with cancer-selective toxicity have received much more attention as alternative chemotherapeutic agents in recent years. However, the basis of their anticancer activity remains unclear. The modification of cell surface glycosylation is a characteristic of cancer cells. The present study investigated the effect of glycosylation, in particular sialic acid, on the anticancer activity of ABPs. We showed that aurein 1.2, buforin IIb and BMAP-28m exhibited selective cytotoxicity toward MX-1 and MCF-7 breast cancer cells. The binding activity, cytotoxicity and apoptotic activity of ABPs were enhanced by the presence of O-, N-glycoproteins, gangliosides and sialic acid on the surface of breast cancer cells. Among N-, O-glycoproteins and ganglioside, O-glycoproteins almost had the strongest effect on the binding and cytotoxicity of the three peptides. Further, up-regulation of hST6Gal1 in CHO-K1 cells enhanced the susceptibility of cells to these peptides. Finally, the growth of MX-1 xenograft tumors in mice was significantly suppressed by buforin IIb treatment, which was associated with induction of apoptosis and inhibition of vascularization. These data demonstrate that the three peptides bind to breast cancer cells via an interaction with surface O-, N-glycoproteins and gangliosides. Sialic acids act as key glycan binding sites for cationic ABP binding to glycoproteins and gangliosides. Therefore, glycosylation in breast cancer cells plays an important role in the anticancer activity of ABPs, which may partly explain their cancer-selective toxicity. Anticancer ABPs with cancer-selective cytotoxicity will be promising candidates for anticancer therapy in the future.

Design of membrane targeting tobramycin-based cationic amphiphiles with reduced hemolytic activity

Hemolysis caused by amphiphilic tobramycin antimicrobial agents is significantly reduced by altering the bond linking their hydrophobic and hydrophilic parts.

High-throughput platform for rapid deployment of antimicrobial agents

A new approach to conducting bacterial binding assays by using an addressable high density random sequence peptide microarray is described. When bacterial binding is carried out in the presence of a competing excess of corresponding bacterial lipopolysaccharide (LPS), most of the observed bacterial binding is inhibited, suggesting that LPS is the major target of the bacterial binding peptides. Importantly, the amino acid composition of the selected peptides closely resembles the composition of natural antimicrobial peptides. Conjugation of selected peptides to polyvalent nanoparticle scaffold yields constructs that show potent antibacterial agglutination activities. The system is general enough to potentially create antimicrobial agents to virtually any pathogen.